This invention relates to a novel screw and matching novel bit. In particular, it relates to a screw having at least two undercut slots at the periphery of its head, and to a bit that engages those slots.
Power screwdrivers are now used more and more, especially when a large number of screws must be fastened. Power screwdrivers can produce greater torque than can a human, but if the rotation of the driver is not stopped immediately when the bottom of the head of the screw hits the surface into which the screw is being driven, both the slot in the screw and the bit can be stripped, destroying the bit and making it difficult to remove the screw. It is not easy to stop the driver just when the head of the screw contacts the surface, so stripped screws and ruined bits are common. This is especially true of Phillips screwdrivers (X-shaped slots) and square drives (where the slot is a square well) because the bit engages the slot so close to the center of the head of the screw that there is very little mechanical advantage. In addition, the torque of the driver tends to force the bit out of the slot, so that the operator must apply considerable pressure to keep the bit in the slot.
U.S. Pat. No. 6,526,851, the grandparent of this application, shows a screw having an undercut slot and a matching bit. The bit can be easily inserted into the slot and, when the bit is turned to tighten the screw, the bit is forced towards the screw and will not easily disengage from the screw in a vertical (z-axis) direction. Also, the bit applies torque to the periphery of the head of the screw, not to the center, thereby reducing the likelihood that the screw and bit will be stripped. However, the bit can still slide out of the slot in a horizontal (x-axis) direction. Also, unlike a Phillips screw, where the bit slides into position as it engages the screw, the bit does not automatically slide into position at the center of the slot and the user must place it in the center and keep it there while tightening the screw.
U.S. patent application Ser. No. 10/248,790, the parent of this application, shows a screw and bit that are similar to that of U.S. Pat. No. 6,526,851, but there is a protuberance in the center of the slot and a matching well in the center of the bit. The protuberance and well guide the bit to the proper position and prevent the bit from sliding out of the slot along its length. While the use of the protuberance and well is an improvement over the screw and bit of U.S. Pat. No. 6,526,851, the screws of both U.S. patent application Ser. No. 10/248,790 and U.S. Pat. No. 6,526,851 are difficult to manufacture using cold heading, the most common technology for making inexpensive screws. Thus, it would be desirable to have a screw and bit with all of the advantages of the screws and bits of U.S. patent application Ser. No. 10,248,790 and of U.S. Pat. No. 6,526,851, but which can be easily made by cold heading.